A typical dispatch two-way radio communication system comprises communication units, communication resources, communication sites, and a communication resource allocator. Each of the sites have a substantially distinct coverage area and are geographically located throughout the system. Each site also has a number of communication resources assigned to it, where at least one of the communication resources is used as a control channel, while a number of the remaining communication resources are used as voice channels. Such systems are known to use both frequency division multiplex access (FDMA) and time division multiple access (TDMA) methods to rebroadcast transmissions.
In a system that supports dispatch group calls, the communication units are typically arranged into communication groups (talk groups) and may be located anywhere within the system (in any site). When a communication unit of a talk group requests a group call, it transmits, via a control channel of the site in which it is located, an inbound signaling message to the communication resource allocator. (A group call typically allows all members of the same talk group that are located within the system to communicate with each other.) The inbound signaling message generally comprises the requesting communication unit's individual identification number, the requesting communication unit's talk group, and a request for a group call. Upon receiving the inbound signaling message, the communication resource allocator will allocate a voice channel in any necessary sites to the requesting communication unit's talk group.
Thus, in FDMA and TDMA dispatch systems a forward link is established (one in each site where communication units are present) and monitored by all units involved in the group call in that site, and a single reverse link which is used by the group member who is currently transmitting to the other members. Non-transmitting talk group members are typically in a listen only mode (i.e., not able to transmit when another member is talking) and thus are not allocated a dedicated reverse link. In a TDMA system, for example, time slots are allocated to different users. A talk group member transmitting on an assigned reverse link, is allowed to use full power transmission to support short bursts of signals in allocated time slots. Despite the discontinuous or discrete nature of this approach, the listener receives what appears a continuous service.
In the last decade, in response to an ever-accelerating worldwide demand for mobile and personal portable communications, spread spectrum digital technology, of which one type is known as code division multiple access (CDMA), has proved to be an alternative for serving large populations of multiple access users over analog or other digital technologies. CDMA relies on processing power to extract a coded signal embedded across a broad frequency spectrum. The only way to extract the wanted signal from among many other overlaid unwanted signals is to have the right code. The use of coding allows more channels to be derived by the overlaying of carriers one over another and greatly enhances performance in terms of derived channels per hertz of bandwidth.
CDMA is well suited for cellular communications, but has never been employed in a system supporting dispatch group calls. The difficulties in applying CDMA techniques to dispatch group communications (such as forward and reverse power control and the need to support soft hand-offs) are discussed in co-pending patent applications "Establishment of Multiple Low-Rate Inbound Signaling Links in CDMA Dispatch System" Ser. No. 08/959,327, "Fast Call Setup In a CDMA Dispatch System" Ser. No. 08/959,152. In addressing these problems, both of these prior applications discuss the establishment of a low-rate (non-voice) signaling link in the reverse direction (inbound). Using such a low-rate inbound signaling link, which signals at a predetermined rate substantially less than that of a full-rate traffic link, proportionately less average power is required for dispatch communications than would otherwise be the case if full-rate traffic links were provided instead to non-talking talk group members. Additionally, it is recognized in these prior applications that such low-rate reverse links can be used to maintain the correct reverse link power control for each non-talking member to allow for fast channel access for any follow-on calls (i.e., group-wide calls initiated by non-talking members following a current group call transmission).
While reverse links for dispatch group calls can be used to facilitate reverse power control, substantial difficulties must be overcome. In particular, it must be recognized that in existing CDMA systems, reverse power control is maintained in accordance with information sent over a forward (outbound) channel. In particular, existing CDMA systems use in-band control signaling (i.e., control signaling embedded in an outbound traffic channel) to continuously instruct individual subscriber units how to adjust their reverse power, typically up to 800 times per second. This method works well for one-to-one type calls. However, this method quickly breaks down for one-to-many type calls. In particular, the amount of information required to continuously update even a modest number of non-talking members in a group call would occupy too much of a given outbound traffic channel's bandwidth, leaving little, if any, bandwidth for the actual outbound traffic. Additionally, because the outbound traffic channel in a group call scenario would not be assigned to a single user, as in one-to-one type calls, it becomes necessary to direct specific reverse power control information to each of the non-talking members.
Thus, there exists a need to provide reliable and efficient reverse power control for group call communications in CDMA systems.